Correspondence Address:Mr. Eric John KruseUniversity of Chicago Medical Center, Chicago, Illinois United States of America

Source of Support: None, Conflict of Interest: None

DOI: 10.4103/jiae.jiae_76_17

Abstract

Three-dimensional echocardiography (3DE) is a new echocardiographic tool that enables echocardiographers visualization of cardiac structures from any anatomical view. Furthermore, the recent development of new transducer technology and software allows the easy acquisition and analysis of datasets for sonographers. A few common applications of 3DE consist of the left and right ventricle for chamber quantification, mitral valve stenosis and regurgitation assessment, and the guidance of catheter placement during interventional procedures. Despite current literature illustrating the importance of 3DE, it fails to demonstrate how to acquire 3D datasets from the sonographer's perspective. Understanding 3DE data acquisition technique and applications are paramount to implement it as standard of care. Acquisition of 3DE should be accomplished in three steps (1) optimization, (2) acquisition, and (3) cropping of 3D images.

Real-time three-dimensional echocardiography (3DE) has been introduced about two decades ago and has been commercially available since the early 1980's. Both transesophageal (TEE) and transthoracic (TTE) 3DE applications continue to develop, providing cardiac sonographers, cardiologists, and surgeons alike with a technology that provides a more accurate diagnosis.[1] The implementation of 3DE using the matrix array transducer in the 1990s has become increasingly developed by all vendors.[2] Although 2DE should be performed routinely, the role of 3DE is vital in the understanding valvular pathology, interventional procedural guidance, and chamber quantification. Recently, the most recent guidelines recommend the use of 3DE for the quantification of the right and left ventricular volumes.[3] Still, the mystery surrounding 3DE implementation appears to be one of the most significant impediments to the routine use of 3DE as a diagnostic tool. Demystifying 3DE should be accomplished in three steps, which demonstrate how easy acquisition of 3D images can become. These steps consist of (1) optimization, (2) acquisition, and (3) cropping of 3D images.

2D Image Acquisition

As a general rule, a poor 2D image results in even poorer 3D images. Therefore, optimization of the 2D image utilizing the focus, gain, and dynamic range is necessary. Placing the focus in the region of interest ensures that the strength of the ultrasound signal is not lost due to attenuation. However, when imaging structures that are deep in the imaging sector there always is an inherent risk of weak returning signals. Utilizing gain and time-gain compensation (TGC) can help nullify attenuation. Increasing the gain and TGCs will strengthen the weak signals which will enable better visualization of the blood-tissue interface whereas decreasing the gain will eliminate low amplitude signals. The appropriate gain settings should be used to ensure that no valuable data are lost while also eliminating unnecessary data. The dynamic range should also be adjusted once the gain and TGCs settings are optimized. Decreasing or increasing the number of gray shades will further enhance the blood-tissue interface. Endocardial definition is essential for accurate 3D analysis of chamber quantification and valvular assessment [Figure 1].

Another consideration when acquiring adequate 3D image is the possibility of acquiring confounding artifacts. During image acquisition, it is common for stitch artifacts [Figure 2] to be introduced, especially when data are acquired with the multibeat acquisition mode.[4] While the multibeat acquisition increases the frame rate, it also requires additional acquisition time. For example, when using a 4 beat acquisition, the sonographer must have the patient hold their breath for at least five cardiac cycles since it will take four cardiac cycles to stitch the data set together to obtain a complete dataset, allowing analysis to be completed during the 5th cardiac cycle [Figure 3]b. Stitch artifacts are common due to translational motion of the heart due to breathing and/or transducer movement. It is important for the imager to have the patient perform a breath hold while acquiring images to minimize transducer motion. It is encouraged to first observe the difference in image quality when the patient slowly takes a breath, then repeat the acquisition while slowly releasing the air. This step allows the imager to observe changes in imaging quality while determining the appropriate amount of air the patient needs to have in the lungs to acquire a good image. In addition, the echocardiographer must maintain a stable position of the transducer to properly assess the influence of breathing on image quality. Failure to maintain the transducer position will result in similar stitch artifact as the one originating from inadequate breath holds. A good electrocardiogram (ECG) tracing is also required while acquiring a multibeat acquisition. The multibeat acquisition is ECG gated and triggered by the R-wave, therefore, irregularities in the R-R interval will result in stitch artifact. This problem is most commonly seen in patients with atrial fibrillation, atrial flutter, and frequent ectopy. To avoid the aforementioned artifacts that occur with multibeat acquisition, the imager could opt for a single beat acquisition when patients have irregular rhythms [Figure 3]a. This acquisition mode allows the imager to only have to wait for a single cardiac cycle to complete an entire dataset. However, the inherent drawback with this acquisition mode includes image degradation from decreased frame rate.

Figure 3: Three-dimensional echocardiographic image acquisition. (a) Single-beat acquistion in which the entire dataset is collected in one cardiac cycle. (b) Multibeat acquisition in which the entire dataset is collected over four cardiac cycles. This acquisition would be referred to as a 4 beat acquisition

The next step in demystifying 3DE is to select the appropriate acquisition mode for the particular case [Figure 4]. The best approach is to first determine what additional information is 3DE dataset acquisition going to accomplish. Separating chamber quantification and valvular assessment into different categories will help narrow down the options. It is also important to remember the differences between a matrix probe and the standard imaging transducer [Figure 5].[5] The matrix probe has the capability of displaying images from two different angles simultaneously. For example, when acquiring the left ventricle using the standard apical four-chamber view (AP4), it is to simultaneously visualize the endocardium of both the AP4 and the AP2 views [Figure 1]. This enables the imager to ensure that the adequate data are acquired.

Perhaps, for beginners, the easiest acquisition mode to start data acquisition is the full-volume method. The full-volume acquisition offers the largest sector width [Figure 4]C with the highest spatial and temporal resolution (>30VPS). Conventionally beginning with the left ventricle using the full-volume method allows the imager to focus on larger region of interest. The real-time or live 3D imaging (narrow-angle) [Figure 4]A is an intermediate acquisition method used to assess a smaller region of interest. The narrow data set can be then steered by adjusting either the lateral or elevation position using the trackball. This mode offers different perspectives without requiring probe manipulation and is often used to guide catheters and wires during interventional procedures such as transcatheter aortic valve replacements (TAVR) and Mitra-clips.[6] The real-time acquisition mode also uses a single heartbeat to instantaneously provide the exact location of the catheters. Nonetheless, the major pitfall of using the real-time acquisition mode is the limited temporal and spatial resolution.

To further assess fast-moving structures such as valves, the 3D-zoom acquisition mode [Figure 4]B provides a smaller but yet detailed data pyramid. In essence, the 3D-zoom mode selects a small section of the complete data set to focus on. The added value is similar to traditional 2D zoom, in that unnecessary data are not included in the complete dataset. However, when compared to the full-volume mode, the 3D-zoom has reduced spatial and temporal resolution.

The last acquisition mode available is color 3D [Figure 6], which can be applied to any of the previously mentioned modes. This mode allows localization of the origin of regurgitant lesions, device positioning during TAVR procedures, and visualization of residual regurgitation jets postMitraClip insertion to name a few. In addition, 3DE improves accuracy when quantitatively assessing regurgitant lesions without the need for geometric assumptions. For example, 3D color vena contracta area in cases of ischemic mitral regurgitation (MR) tends to be eccentric rather than circular.[7] Therefore, using traditional 2D vena contracta results in an underestimation of the effective regurgitant orifice area. Currently, color 3DE is limited do to the inherited frame rate reduction that occurs. However, recent advancements in color 3DE from the TEE approach have shown promise.

Figure 6: This image demonstrates a utilization of color three-dimensional transesophageal in a multi-image layout during a septal defect closure

Assessing the left ventricle is an essential component of every echocardiographic study that is ordered. Therefore, accurate quantitative assessment of the size and function of the left ventricle is vital.[8] TTE 3DE of the left ventricle can provide volumes, global and regional function, LV mass, and shape.

According to the current ASE chamber quantification guidelines, a biplane Simpson's method of discs is recommended to measure left ventricular volumes and ejection fraction.[3] In addition, careful imaging of the left ventricle is required to avoid foreshortening when using 2DE. The advent of 3DE alleviates the risk of foreshortening and eliminates geometric assumptions.[9] Standard 2DE requires the imager to accurately pass the 2D plane through the true apex of the left ventricle in multiple views. However, when using 3D multiple plane reconstruction, the imager can manipulate the dataset to ensure that the true apex is visualized in all apical views.

Sonographer approach to the left ventricle

Imaging the left ventricle first requires the sonographer to align the left ventricle in the middle of the imaging sector in the AP4 window

Ensure that the entire left ventricle endocardium is visualized throughout the cardiac cycle by adjusting 2D gains, compression, and decreasing the sector width. When assessing the left ventricle, it is recommended to exclude the right heart and both atria

Utilize the biplane mode two ensure that both the AP4 and AP2 have adequate images including proper endocardial definition throughout the cardiac cycle

Select the full-volume acquisition mode utilizing four beats and change the image layout to the quad screen which will minimize the presence of stitch artifacts

Have the patient pause respirations and wait for at least six cardiac cycles. It is recommended to freeze the image two beats after the dataset has been stitched together

Replay the image to ensure that no translation of the heart and/or transducer motion has occurred during acquisition.

The Right Ventricle

Similarly, to the left ventricle, the right ventricle is assessed routinely in clinical echocardiography. Once deemed to be the “forgotten ventricle,” assessment of the right ventricular (RV) ejection function, and size is becoming essential in routine studies.[10] However, accurate assessment the complex geometry of the right ventricle can be difficult. In addition, the infundibular segment of the right ventricle which involves 25%–30% of the RV is frequently not visualized using traditional 2DE views [Figure 7]. Therefore, routine use of 3DE to assess the right ventricle is recommended.[11]

Figure 7: Short axis of the right ventricle. This figure represents the infundibular segment of the right ventricle using three-dimensional echocardiography, not accounted for with traditional two-dimensional echocardiography

Obtain an RV focused view by sliding the transducer laterally and placing the right ventricle into the middle of the imaging sector. It is recommended to obtain the RV focused view when acquiring the 3D RV dataset to minimize the possibility of the right ventricular free wall dropout

Optimize the right ventricular images by adjusting gain, compression, and focus

Select the full-volume acquisition mode to include the entire right ventricle in the imaging sector

Select the appropriate number of beats accordingly to the individual patient. It should be noted that single beat acquisition usually results in low of frame rates for postprocessing analysis

In addition to adjusting the depth of the image, narrow both the lateral and elevation widths to exclude unnecessary data.

3DE in Mitral Valve Regurgitation

Imaging the mitral valve (MV) using traditional 2DE from both the TEE and TTE approach has proven to be clinically useful. However, identification of the particular leaflet involvement using 2DE is cumbersome and time-consuming when compared to 3DE TEE.[12] The added advantage of 3DE includes the third-dimension depth, allowing the imager to visualize the MV in its entirety as well as the surrounding structures. Simply put, traditional 2DE provides data from left to the right and up to down, in a thin slice of the heart. Modern 3DEmultiplanar data not only provides data similar to that of traditional 2DE but also includes data from the front of the image to the back (depth). Understandably, having a thicker slice, using 3DE, allows the imager to visualize structures moving toward or away instead of in or out of the imaging plane.[13] Datasets derived from 3DE allow the imager to visualize the MV from either the atrial or ventricular perspective [Figure 8]. This becomes particularly useful in the assessment of MV prolapse or flail.[14] More specifically, when viewing the MV from the atrial perspective, imagers can identify which scallop/s are the causes of MR.[15] Recent studies have also suggested the added value of MR quantification using color 3DE for allowing visualization and measurement of the vena contracta area and 3D proximal isovelocity surface area (PISA).[16] Manipulation of 3D color datasets can accurately quantify and locate the MR jets.

Figure 8: Assessment of the mitral valve from the atrial perspective using three-dimensional transesophageal echocardiography

In addition to identifying leaflet involvement of functional and/or secondary MR, 3DE is very useful during interventional procedures. Imagers can visualize wires and catheters during Mitra-clip procedures, in fact, 3DE has become critical to ensure a successful interventions. Utilizing the biplane mode can accurately show the interventional cardiologist precisely where the clip should be placed, relative to the regurgitant lesion. The imager can also ensure that the clip is not negatively interfering with the chordal apparatus. Improper deployment of the clip can result in worsening MR or in some instances create mitral stenosis (MS).

Sonographer approach to the mitral valve

Ensure there is a clean ECG tracing with a consistent R-R interval

Select the window that allows for best visualization of the MV

Optimize 2D image settings

Select the 3D-zoom acquisition mode

Change beat acquisition

Narrow lateral and elevation width to optimize frame rate (>18 Hz)

Pause respirations for the duration of the dataset acquisition

Acquire replay dataset to check for stitch artifacts.

3DE in Mitral Stenosis

Rheumatic MV stenosis is a disease which requires accurate echocardiographic and clinical evaluation. Until now, traditional 2DE methods to assess MS include 2D planimetry, pressure-half-time, and PISA which provide valuable insights to the severity of MS. However, finding the optimal plane of the smallest MV orifice is a limitation that affects the accuracy of MV area quantification. 3DE is necessary to accurately quantitatively assess the degree of MS.[17] The inherent benefit of 3DE is that the imager can manipulate the dataset to ensure that the imaging planes intersect the exact location of the region of interest.[18] In terms of measuring MS using the parasternal long axis (PLAX) view, the data set should be displayed to visualize the PLAX in its orthogonal plane. Adjustments should be made to align one plane through the narrowest orifice in the PLAX allowing the imager to measure the planimetry of the MV in the orthogonal plane [Figure 9].

Figure 9: Biplane imaging of the mitral valve. The parasternal long axis view is utilized to visualize the mitral valve in the orthogonal imaging plane for measuring the mitral valve area

Imaging the MV in patients with MS from the TTE perspective should be obtained from the PLAX and/or apical four-chamber (AP4) views. When selecting to use the TEE approach, the apical three chamber (130) mid-esophageal level is recommended to best visualize the MV. The 3D-zoom or full-volume acquisition modes can also be used to quantify MS. The most important aspect to consider when deciding between the available views and acquisition modes is image quality.

Sonographer approach to mitral stenosis

Ensure a clean ECG with a consistent R-R interval

Select the appropriate window to best visualize the MV

Optimize the 2D image using gain, compression, and focus

Select 3D-zoom or full-volume acquisition mode

Decrease lateral and elevation width

Select quad-screen image layout

Adjust beat acquisition and have the patient pause respirations

Acquire image and replay to check for stitch artifact

Select image to perform postprocessing measurements

Display image in MPR mode

Align imaging plane to intersect the narrowest MV orifice in diastole

Planimeter the MV using orthogonal plane.

Aortic Valve

Detailed assessment of the aortic valve using 3DE is difficult in healthy individuals due to the thin makeup of the leaflets. However, the benefits of 3DE in patients with diseased aortic leaflets are well documented. In particular, the assessment of aortic stenosis becomes more accurate and reproducible. 3DE provides imagers with the ability to align the cropping plane parallel to a specific region of interest. The left ventricular outflow tract (LVOT) can then be planimetered to use for the continuity equation when calculating the aortic valve area (AVA). Traditional 2DE calculations of the AVA using the continuity equation typically results in underestimation, assuming the LVOT is circular. In addition, aortic root measurements for procedural planning can also be performed using reconstructed images.[19]

Sonographer approach to the aortic valve

Ensure there is a clean ECG tracing with a consistent R-R interval

Select the window that allows for best visualization of the AV

Optimize 2D image settings

Select the 3D-zoom acquisition mode

Change beat acquisition

Narrow lateral and elevation width to optimize frame rate (>18 Hz)

Pause respirations for the duration of the dataset acquisition

Acquire replay dataset to check for stitch artifacts.

Tricuspid Valve

Imaging the tricuspid valve with standard 2D echocardiography usually results in just two of the three leaflets visualized. However, 3DE allows imagers to display all three leaflets of the tricuspid valve in a single plane.[20] Moreover, imaging of the tricuspid valve is best performed using either the right ventricular inflow or apical four views using the TTE approach. The mechanism of tricuspid regurgitation or leaflet (s) involvement in the pathology can now be easily pinpointed.

Sonographer approach to the tricuspid valve

Ensure there is a clean ECG tracing with a consistent R-R interval

Select the window that allows for best visualization of the tricuspid valve (TV)

Optimize 2D image settings

Select the 3D-zoom acquisition mode

Change beat acquisition

Narrow lateral and elevation width to optimize frame rate (>18 Hz)

Pause respirations for the duration of the dataset acquisition

Acquire replay dataset to check for stitch artifacts.

Cropping/image Layout

The last step to demystify 3DE is to crop the 3D datasets that have been acquired. Cropping of 3D datasets depend on the structure being viewed and the chamber that needs to be quantified. Typically, cropping is done from the x-, y-, or z- axis planes [Figure 10]. These plane adjustments allow the imager to eliminate data from the top/bottom (y-axis), left/right (x-axis), and front/back (z-axis) of the image. Often, the imager needs to use multiple cropping planes to better appreciate the region of interest. Imagers can visualize a thicker portion of a moving structure and highlight a specific region of interest from any perspective. The thickness of the image is created by the third dimension (z-axis), which gives the imager the perception of depth. For example, when evaluating a MV prolapse from the left atrial perspective using 3D TEE, the imager can visualize the specific prolapsing scallop of the MV [Figure 7]. Contrary to standard 2D echocardiography, 3DE allows imagers to use a much larger dataset to eliminate structures from moving in/out of the imaging plane.

Three of the most commonly used image adjustments are the 3D gain, compression, and brightness. Conventionally, it is best to begin with 3D gain, as this either adds or eliminates lower received signals. There are two approaches to consider when adjusting 3D gain. The imager may begin by reducing the 3D gain to create dropout and then slowly increase gain or begin with the highest level of 3D gain and slowly reduce the 3D gain “CROP TILL YOU DROP.” Once the appropriate level of 3D gain is determined, compression should be optimized. 3DE compression is similar to 2DE; however, instead of black and white images with 2DE, the 3DE compression displays them in color. The 3DE compression can be adjusted to include either a wider or narrow range of the color shades. The 3DE image dataset can then be adjusted for the level of brightness although considered cosmetic, image brightness helps increase the intensity of the higher received signals.

Cropping and layout of left ventricle

Acquire Multibeat LV full volume

Select quad-screen image layout

Turn dataset 90° around the x-axis to view the left ventricle from the apex

Rotate the dataset 90° clockwise (right ventricle on top of left ventricle)

Ensure that stitch artifact are not present

Adjust 3D Gain, compression, and brightness

Quantify using available software.

Cropping and layout of right ventricle

Acquire Multi-Beat RV full-volume

Select quad-screen image layout

Turn dataset 90° around the X-axis to view the right ventricle from the apex

Rotate the dataset 90° clockwise (right ventricle on top of left ventricle)

3D echocardiography has many applications and provides echocardiographers with critical information required to accurately diagnose heart disease. A few of these applications include left and right ventricle chamber quantification, which have been described in the current ASE chamber quantification guideline recommendations. In addition, 3D echocardiography is proven to be an essential tool to localize specific regurgitant valvular lesions. Furthermore, 3DE for the guidance of catheter placement during interventional procedures such as balloon valvuloplasty, atrial septal punctures, TAVRs, and MitraClips has increased significantly in recent years. However, the added benefits of 3DE are not routinely utilized due to in part the mystery that continues to surround it. To demystify 3DE, we should simply divide 3DE into three steps, image optimization, 3D mode of acquisition, and cropping. Although technology continues to improve the quality and accessibility of 3DE, daily utilization of 3DE in echocardiographic laboratories will ultimately result in demystification of 3DE for sonographers and physicians alike.